#include<stdio.h>
#include<stdlib.h>
char * strcat(char* src,char* str2);
void main(){
	char a[100] = {NULL};
	char *b = NULL;
	printf("%s\n",strcat(a,b));
}
char * strcat(char* src,char* str2){
	if(src==NULL&& str2 == NULL){
		return NULL;
	}
	if(src==NULL)return str2;
	if(str2 == NULL) return src;
	char * addr = src;
	while(*src)++src;
	while(*src++ = *str2++);
	*src = '\0';
	return addr;
}